1. Field of the Invention
The present invention relates to an auto focus system, and in particular, to the auto focus system for controlling a focus of a picture-taking lens by a contrast method.
2. Description of the Related Art
An auto focus control of a video camera and so on is generally depending on a contrast method. According to the contrast method, high frequency components of picture signals in a certain range (focus area) of the picture signals obtained from an image pickup device are integrated to be a focus evaluation value, and a focus adjustment is automatically performed so that the focus evaluation value becomes maximum (local maximum). It is thereby possible to obtain the best focus (focusing) for maximizing a degree of sharpness (contrast) of an image picked up by the image pickup device.
A so-called mountain climbing method is widely known as a method of setting a focus at a focusing position (a local maximum point of the focus evaluation value). According to this method, a direction in which the focus evaluation value increases is determined by comparing the focus evaluation values at two different points on moving the focus, and the focus is moved in that direction so that if the focus evaluation value turns to decrease from increase, the focus is returned to the position before the focus evaluation value decreased so as to set the focus at the local maximum point of the focus evaluation value.
In the case of the above-mentioned mountain climbing method, there is a drawback that an increasing direction of the focus evaluation value and focusing cannot be determined without actually moving the focus. Therefore, there is a proposed method whereby a focus state (front focus, rear focus or just focus) of a picture-taking lens is determinable without moving the focus by placing a plurality of image pickup devices at positions of different light path lengths (e.g., WO 02/099495 A1, WO 02/099496, WO 02/099497 A1 and WO 02/099498 A1, which were, at the time the present invention was made, not published and not publicly known). According to this focus state determination method, it is possible to immediately know a current focus state from a magnitude relation among current focus evaluation values obtained from the image pickup devices so as to determine a moving direction of the focus and the just focusing without moving the focus. Therefore, the auto focus control using this method has advantages such as being able to promptly set the focus at the focusing position.
In case of exerting auto focus control by using a plurality of image pickup devices as mentioned above, it is necessary, in order to improve accuracy of the auto focus control, to match up sensitivity of focus evaluation values (relationship between object light incident on each image pickup device and size of the focus evaluation value obtained therefor) obtained from each of the image pickup devices. To be more specific, the auto focus control is exerted on the assumption that characteristics of the image pickup devices and processing circuits for processing picture signals from the image pickup devices match up. To be precise, however, there are variations in the characteristics of the image pickup devices and processing circuits, and so it is important, for the sake of improving the accuracy of the auto focus control, to make an advance correction on the sensitivity of the focus evaluation values obtained from the image pickup devices.
The focus evaluation values can be obtained by performing the same process to the picture signals from a plurality of focus state determining image pickup devices, respectively. However, if that process is performed in parallel by the processing circuits provided corresponding to the picture signals, images of the same time are obtained from the image pickup devices, and there is a drawback that a flicker of a fluorescent lamp may occur and the circuit size and power consumption are increased although focus information from an object moving on the screen can be processed with high accuracy.